1. Field of the Invention
This invention relates in general to signal processing and, in particular, to such signal processing as may be employed in connection with an "off-the-air" color video recorder. While not restricted to the recording of signal information on magnetic tape, the invention is cast in such an environment to facilitate its description.
2. Description Relative to the Prior Art
The relatively short history of video recording, whether off-the-air or otherwise, would indicate a trend toward complexity, and away from simplicity. Unlike the audio bandwidth -- which may easily be recorded directly, i.e., without a carrier frequency -- the video bandwidth presents a severe challenge to a recording system. The video spectrum has a range of about 17 octaves (versus about 10 for sound) and extends to about 4.5 MHz (versus only about 15,000 Hz for sound). Attendantly, the recording of video information has primarily been via modulation techniques, whereby the number of octaves to be recorded can be appreciably reduced. FM recording, in particular, has been the vehicle by which most video recording has been implemented, a typical FM carrier for the baseband video being, say, 12 MHz. Considering that both sidebands are recorded (which is not always the case), this would suggest that the bandpass requirements of such a system would be from 7.5 MHz to 16 MHz, which is only about one octave.
While the octave-reducing quality of FM recording is desirable, it is achieved not without cost, viz., that of system complexity. To replay, say, a 16 MHz signal which is recorded on magnetic tape means that either the head-gap used for playback be extremely small, or that the head-to-tape speed be extremely fast. Typically, with head gaps of 50 microinches, head-to-tape speed has had to be about 1500 inches per second (ips). In light of such high head-to-tape speeds, the art has resorted to the use of so-called rotary systems (transverse; helical), whereby tape transport speed may be kept reasonably slow. Notwithstanding the trend toward modulation of the video signal prior to recording, the head-to-tape speed for a given head gap may be reduced appreciably if the video signal is recorded directly, i.e., without a carrier frequency. Thus, direct recording continues to hold interest for many in the video recording field, provided, of course, that the number of recorded octaves can be reduced sufficiently and without adversely affecting the quality of the images which result from such recording.
As indicated above, the invention is concerned with "off-the-air" recording of video signal information, typically an NTSC (or PAL) signal: The NTSC signal, as is known, contains luminance signal information from DC up to about 4 MHz, and that color signal information (chroma) is interleaved with the luminance signal information at the upper end of the luminance spectrum. Since the acuity of the eye is incapable of resolving, under normal viewing conditions, luminance detail corresponding to luminance signals above about 3 MHz, television manufacturers have, for sake of economy, conventionally bandlimited luminance channels to somewhere between 2 to 3 MHz, thereby avoiding the problem of having to comb luminance signal information out of the interleaved chroma/luminance part of the NTSC spectrum. Such a practice poses the suggestion that bandwidth compression for video recording purposes be implemented by interleaving the chroma information with the luminance information below about 3 MHz provided, of course, that the interleaved chroma and luminance information can be combfiltered apart during playback. Given the relatively simple task of interleaving laboratory-produced chroma and luminance, this general approach has already been implemented. See, for example, SID 1975 Digest, pages 14 and 15, which teaches that the spread of the chroma and luminance sidebands around multiples of the video scanning frequency may be reduced, by combfiltering both chroma and luminance prior to signal interleaving, to avoid interaction between the luminance and chroma information. Such a practice may be likened to "sharpening the teeth of two overlaid combs, the respective teeth of which are almost aligned".
Off-the-air recording, on the other hand, is by no means a laboratory setup, since there is no control over the source of the video signal: The NTSC signal is customized, standardized, and designed not for a video recorder, but rather for reception and processing by a television receiver.
FIG. 1, which is described in detail below, discloses an off-the-air video tape recording system of the direct recording type in which the NTSC signal is detected; the chroma informaton (0.5 MHz bandwidth) removed from the detected signal; and then down-converted from its conventional subcarrier frequency (3.58 MHz) so as to interleave with luminance information bandlimited to about 2.5 MHz. During playback, the chroma and luminance information are combed apart; the chroma up-converted to 3.58 MHz; and the 0 to 2.5 MHz luminance information, and up-converted chroma centered at 3.58 MHz (.+-. 0.5 MHz), which together comprise a modified NTSC signal, placed on a carrier for replay through a vacant channel of a television receiver.
Visual examination of the television picture, produced using the setup of FIG. 1, indicated a fairly acceptable picture, but for certain liveliness (not Moire) appearing within the picture: The source of such lively artifacts thus had to be identified so that appropriate measures could be taken to remove the problem source. Otherwise, off-the-air recording of interleaved luminance/chroma, derived from an NTSC signal, would not be practical in a commercial sense. At first blush, it had appeared that any one, or combination, of the components of FIG. 1 could be the source of the above-identified artifacts: The antenna system could have been bad, the tape could have been noisy; or any modulator, or oscillator, or amplifier, etc, could have been working improperly, e.g., non-linearly.
Rather than one of the more obvious sources of the picture liveliness problem, it was found -- as disclosed in copending U.S. application Ser. No. No. 605,101 -- that a fundamental characteristic of the NTSC signal had to be taken into consideration if the direct recording of interleaved chroma and luminance signal information were to be effective, viz., while it is true that the power contained in luminance information at around 3.58 MHz is quite low, it is nevertheless not nonexistent. Any attempt to down-convert chroma at 3.58 MHz, as indicated in connection with FIG. 1, resulted in the simultaneous down-conversion of low power luminance signal information -- and it was the down-converted luminance signal information which, when it beat against the lower frequency luminance components, caused the picture liveliness which was noted. Accordingly, as indicated in copending application Ser. No. 605,101, the off-the-air received NTSC-type signal was pre-combed of residual high frequency luminance information prior to its being downconverted, thereby avoiding the above-noted beating of high and low frequency luminance components; and resulting, during playback, in a television picture in which picture liveliness was minimized.
While picture liveliness was minimized by the teaching of application Ser. No. 605,101, the television picture, however, still left something to be desired: Along horizontal edges within the television picture, a moving picture defect, somewhat akin to what one might experience while looking at a "shooting gallery", was noted. By analysis, as indicated in copending application Ser. No. 605,100, it was determined that the source of the shooting gallery effect was the bleeding -- at the point within the FIG. 1 apparatus where the interleaved chroma and luminance are combed apart during playback -- of chroma information into the luminance channel; and such bleeding resulted from vertical variations in the hue of the recorded scene. To reduce the effect of scene vertical hue variations, copending application Ser. No. 605,100 teaches, in a recording system of the type described, the line averaging of color information, prior to recording such information, as a vehicle for reducing the amplitude and visual effect of the color information which bleeds into the playback luminance channel.
Despite the measures of copending application Ser. Nos. 605,101 and 605,100 to improve picture quality, however, some liveliness in the playback picture still remained.